1. Technical Field
The present invention relates to a method of manufacturing an optical fiber base material and an apparatus therefor. Specifically, the present invention relates to a manufacturing method applicable to manufacture an optical fiber material by utilizing flame hydrolysis and a manufacturing apparatus therefor.
2. Related Art
A manufacturing apparatus by using the VAD method has been known as an apparatus for manufacturing an optical fiber base material. In that apparatus, a quartz base material 2 is suspended in a quartz chamber 1 as shown in FIG. 1, and soot is deposited the quartz base material 2 to acquire a soot deposition 3. A core burner 4 toward the tip of the bottom end of the quartz base material 2 and a cladding burner 5 toward the side surface of the quartz base material 2 are provided, respectively as burners for compounding the soot. Each of the burners is supported by a burner fixing section including a burner holder 6 and a fixture 7 thereof.
In addition to a material line for such as silicon tetrachloride (SiCl4) being a core material, germanium tetrachloride (GeCl4) as dopant for controlling the refractive index, a gas line for such as H2 gas and O2 gas for oxyhydrogen flame is connected to the core burner 4. Meanwhile, a gas lines for such as silicon tetrachloride gas and oxyhydrogen gas are connected to the cladding burner 5.
Each of the core burner 4 and the cladding burner 5 are supported by the fixture 7. Thereby the position of each burner can be adjusted such that the relative position between the burner and the tip of the soot deposition is optimized.
The optical fiber base material is manufactured by the following steps: splaying a source gas from the core burner 4 and the cladding burner 5 to the targeted portion of the quartz base material 2 as rotating the quartz base material 2; depositing soot generated by the flame hydrolysis on the quartz base material 2 in the axial direction; pulling up the quartz base material 2 dependent on the growth thereof, thereby the soot deposition 3 including a core with higher refractive index and a cladding with lower refractive index can be obtained; and dehydrating and transparently vitrifying the soot deposition 3 by a electric furnace.
For the manufacturing method of compounding an optical fiber glass material, the relative positional relationship between the burner and the soot deposition is significantly important because not only it affects the sticking rate and the density of soot but also it is the factor that determines the refractive index profile of the acquired glass base material. Particularly, in the case of the VAD method, since the refractive index profile of the acquired glass base material is varied delicately when any displacement is generated in the relative positional relationship between the burner and the tip of the soot deposition, various methods have been proposed in order to fix the burner.
For example, a molding method that fixes the burner to the base with resin or gypsum as means for solving the displacement of the burner because of a lack of the grasp has been proposed in Japanese utility model Application Publication No. 03-22256.
In addition, a method for improving the fixing accuracy and the reproducibility of the burner by defining the accuracy of the hole for holding the burner and by using the burner fixture employing a fitting method has been proposed, for example, in Japanese Patent Application Publication No. 2000-256032.
However, recently it has been desired to enlarge the optical fiber glass base material, therefore, the amount of the source gas supplied has to been increased. Then, a problem has been generated that the burner is displaced and the fixed portion is loosed because the burner and the burner fixing section are thermally expanded due to the heat cycle for which the soot is deposited and the deposition is stopped so that the relative positional between the burner and the soot deposition is displaced. Moreover, another problem has been generated due to the displacement that the sticking rate, the density and the refractive index profile of the soot in the axial direction of the base material are changed.
Thus, an advantage of the present invention is to provide a manufacturing method for acquiring an optical fiber base material having a stable refractive index profile by constantly keeping the relative position between the burner and the soot deposition and an apparatus therefor. Particularly, an advantage of the present invention is to provide a manufacturing method for stabilizing the position of the burner after the relative position between the burner and the soot deposition is adjusted without changing over time during depositing the soot and an apparatus therefor.
To obtain the above-described advantages, a first aspect of the present invention provides a method of manufacturing an optical fiber base material comprising depositing soot generated by flame hydrolysis by using a burner which generates a flame to compound the optical fiber base material. The fluctuation band of the surface temperature of a burner fixing section that positions the burner is kept equal to or less than 80 degree C. Here, it is preferred that the fluctuation band is lesser, of course, and particularly, the refractive index profile of the resultant optical fiber base material is sufficiently stabilized if the fluctuation band is equal to or less than 80 degree C. as described in detail later.
In addition, according to an embodiment, the temperature of the burner fixing section is controlled by a temperature control mechanism in the manufacturing method. Thereby mechanism that keeps the fluctuation band of the surface temperature of the burner fixing section can be provided.
Moreover, according to another embodiment, the burner fixing section includes a burner holder and a fixture thereof, and the temperature control mechanism includes a heat shield plate arranged between the burner flame and at least one of the burner holder and the fixture in the manufacturing method. Thereby the radiant heat of the burner flame is shielded, so that the surface temperature of the burner fixing section can be prevented from being changed.
According to further another embodiment, the temperature control mechanism includes at least one of a temperature control mechanism that heats the burner fixing section and a temperature control mechanism that cools the burner fixing section in the manufacturing method. Here, the “temperature control” may include “heating”, “cooling” and “keeping the temperature”. “Heating” may include directly or indirectly heating with such as an electric heater. “Cooling” may include cooling with refrigerant. Thereby the surface temperature of the burner fixing section can be kept within a predetermined range.
A second aspect of the present invention provides, in a method of manufacturing an optical fiber base material including a step of depositing soot generated by flame hydrolysis by using a burner which generates a flame to compound an optical fiber base material, a manufacturing apparatus including a temperature control mechanism that keeps the fluctuation band of the surface temperature of a burner fixing section which positions the burner equal to or less than 80 degree C. Here, it is preferred that the fluctuation band is lesser, of course, and particularly, the refractive index profile of the resultant optical fiber base material is sufficiently stabilized if the fluctuation band is equal to or less than 80 degree C. and the amount of fluctuation of all over the optical fiber base material is not beyond the predetermined bounds of permissibility.
According to another embodiment, the burner fixing section includes a burner holder and a fixture thereof, and the temperature control mechanism includes a heat shield plate arranged between the burner flame and at least one of the burner holder and the fixture in the manufacturing apparatus. Thereby the radiant heat of the burner flame is shielded, so that the surface temperature of the burner fixing section can be prevented from being changed.
According to further another embodiment, the temperature control mechanism includes at least one of a temperature control mechanism that heats the burner fixing section and a temperature control mechanism that cools the burner fixing section in the manufacturing apparatus. Here, the “temperature control” may include “heating”, “cooling” and “keeping the temperature”. “Heating” may include directly or indirectly heating with such as an electric heater. “Cooling” may include cooling with refrigerant. Thereby mechanism that keeps the surface temperature of the burner fixing section within a predetermined range can be formed.
Here, all necessary features of the present invention are not listed in the summary of the invention. The sub-combinations of the features may become the invention.
According to the present invention, the position of the burner after the relative position between the burner and the soot deposition is adjusted can be stabilized not only during depositing the soot but also between each manufacturing step of the soot deposition. Thereby an optical fiber base material having a stable refractive index profile can be manufactured.